Nanowire structural evolution from Fe3O4 to ε-Fe2O3

The epsilon-Fe2O3 phase is commonly considered an intermediate phase during thermal treatment of maghemite (gamma-Fe2O3) to hematite (alpha-Fe2O3). The routine method of synthesis for epsilon-Fe2O3 crystals uses gamma-Fe2O3 as the source material and requires dispersion of gamma-Fe2O3 into silica, and the obtained epsilon-Fe2O3 particle size is rather limited, typically under 200 nm. In this paper, by using a pulsed laser deposition method and Fe3O4 powder as a source material, the synthesis of not only one-dimensional Fe3O4 nanowires but also high-yield epsilon-Fe2O3 nanowires is reported for the first time. A detailed transmission electron microscopy (TEM) study shows that the nanowires of pure magnetite grow along [111] and < 211 > directions, although some stacking faults and twins exist. However, magnetite nanowires growing along the < 110 > direction are found in every instance to accompany a new phase, epsilon-Fe2O3, with some micrometer-sized wires even fully transferring to epsilon-Fe2O3 along the fixed structural orientation relationship, (001)(epsilon-Fe2O3) parallel to(111)(Fe3O4), [010](epsilon-Fe2O3)parallel to < 110 >(Fe3O4). Contrary to generally accepted ideas regarding epsilon phase formation, there is no indication of gamma-Fe2O3 formation during the synthesis process; the phase transition may be described as being from Fe3O4 to epsilon-Fe2O3, then to alpha-Fe2O3. The detailed structural evolution process has been revealed by using TEM. 120 degrees rotation domain boundaries and antiphase boundaries are also frequently observed in the epsilon-Fe2O3 nanowires. The observed epsilon-Fe2O3 is fundamentally important for understanding the magnetic properties of the nanowires.